This invention relates to a solid state bonding process of malleable metals and more particularly to a solid state bonding process of malleable metals employing the isothermal solidification of a liquid interface to provide a reliable bond with certain material combinations where unattainable in the past. The term metals as used herein includes alloys.
Some metal material combinations do not normally form a suitably stable solid state bond utilizing standard cold bonding techniques, especially when subsequent high temperature heat treating is required. The cladding of silver to copper or of some silver alloys to some copper alloys are examples of such combinations. The bond developed between these sets of materials may be successful but any attempt to heat them above 800.degree.F after bonding most always results in severe bond blistering and delamination.
In many applications a desirable material combination is one which necessitates a post bonding anneal over 800.degree.F in order to possess certain properties. For example, a three layer composite of a silver or silver alloy bonded to a copper or copper alloy bonded to a high work hardening, high melting point backing material is especially useful as an electrical contact material. However, in order to achieve the necessary formability characteristics in the high work hardening layer of this aforementioned composite, a high temperature heat treatment is required. This heat treatment requirement prevents the use of the standard solid state cold bonding technique and thereby necessitates the use of a more costly alternate bonding method.
It is an object of the present invention to provide a method of solid state bonding of malleable metals which employs the isothermal solidification of a liquid interface. Another object of this invention is to provide a method of cold solid state bonding which creates a composite material of a silver or silver alloy, a copper or copper alloy, and a work hardening backing material which can be heat treated without deleterious effects to the bond integrity. Other objects and features will be in part apparent and in part pointed out hereinafter.
Briefly, the present invention comprises a solid state bonding process of malleable metals employing the isothermal solidification of a liquid interface to overcome bonding problems inherent in certain material combinations when subjected to post-bonding high annealing temperatures. The process employs the use of a solid metal interliner clad between the two components to be bonded which has a lower melting point than the adjacent components, is diffusible into either one or both of the adjacent components, and maintains ductility after alloying with the adjacent components. The composite is then further processed and heat treated at such a temperature to be above the melting temperature of the interliner. During this heat treating anneal, the interliner momentarily liquifies and then isothermally solidifies due to the diffusion of the liquid into either one or both of the adjacent components thereby forming a progressively higher melting point alloy. This new alloy at the bond interface supplies a strong intermediate layer between the two components which is strong and ductile.